1. Field of the Invention
The present invention relates to friction members having an excellent bond strength and a high resistance to rusting, and to a method of manufacturing such friction members. More particularly, the invention relates to friction members, and a method of manufacture thereof, which are highly suitable for use in such applications as disk brake pads, drum brake shoes and clutch disks in brakes and clutches for automobiles, industrial machinery and other equipment.
2. Prior Art
Friction members used as disk pads for disk brakes and related applications in equipment such as automobiles are typically manufactured by placing a friction material preform on an iron or aluminum back plate that has been degreased, surface treated and coated with adhesive, then molding the preform in this state within a mold and subsequently heat-treating, spray-painting, baking and grinding so as to give a finished friction member in which the friction material is bonded to the surface of the back plate.
When rust forms on the surface of the metal back plate in such a friction member, the bond strength between the plate and the friction material decreases.
Surface treatments of the back plate that have been proposed to overcome this problem include the formation of a 5 to 20 xcexcm thick metal (iron, zinc, calcium, manganese) phosphate coating (JP-A 5-346129), a calcium zinc phosphate coating having a coating weight of 2 to 6 g/m2 (JP-A 7-293615), or a 0.2 to 2 xcexcm thick iron phosphate coating (JP-A 11-13802).
However, zinc phosphate-type coatings have a large crystal size and are quite thick, making them subject to a decline in bond strength, particularly at high temperatures. Iron phosphate-type coatings also have major drawbacks. In addition to having a poor rust preventability, they are non-crystalline. As a result, the coatings are thin and scratch easily, so that process control is difficult.
An additional consideration is that, during brake or clutch operation, kinetic energy is converted to thermal energy due to friction, generating high temperatures and repeatedly subjecting the friction member to high thermal loads. Such thermal loading in friction materials is rising ever higher as the speed and performance of automobiles continues to increase. A desire thus exists for friction members which have a good heat resistance and are not subject to a decline in adhesion even with repeated and intense thermal cycling.
Therefore, the object of the present invention is to provide friction members which have a high resistance to rusting, excellent adhesion and heat resistance, a long service life, and can be produced with minimal sludge formation and easy process control. Another object of the invention is to provide a method of manufacturing such friction members.
I have found that high-quality friction members endowed with high resistance to rusting and excellent adhesion and heat resistance can be obtained by forming a zinc phosphate or calcium zinc phosphate conversion coating having a small crystal size, an optimized crystal shape, and a low coating thickness, and thus a low coating weight, between a back plate and a non-asbestos friction material which is bonded to a surface of the back plate and produced by molding and curing a composition made up primarily of a fibrous base, a binder and a filler. In addition, I have also found that because the conversion treatment agent has a high reactivity, treatment of relatively short duration is possible at low temperatures, resulting in minimal sludge generation and easy process control.
Accordingly, in a first aspect, the invention provides a friction member having a back plate; a non-asbestos friction material bonded to a surface of the back plate and produced by molding and curing a composition that includes a fibrous base, a binder and a filler; and a metal phosphate conversion coating formed between the back plate and the non-asbestos friction material. When new, the friction member has a breaking load, as measured in accordance with JIS D4422, of at least 9 MPa, and has a percent bonded surface area, defined as the ratio of the surface area of friction material attached to the back plate surface after shear to the surface area of friction material attached to the plate surface before shear, of at least 90%.
In a second aspect, the invention similarly provides a friction member having a back plate; a non-asbestos friction material bonded to a surface of the back plate and produced by molding and curing a composition that includes a fibrous base, a binder and a filler; and a metal phosphate conversion coating formed between the back plate and the non-asbestos friction material. After a 24-hour thermal history at 300xc2x0 C., the friction member according to this aspect of the invention has a breaking load, as measured in accordance with JIS D4422, of at least 6 MPa, and has a percent bonded surface area, defined as the ratio of the surface area of friction material attached to the back plate surface after shear to the surface area of friction material attached to the plate surface before shear, of at least 50%.
In a third aspect, the invention similarly provides a friction member having a back plate; a non-asbestos friction material bonded to a surface of the back plate and produced by molding and curing a composition that includes a fibrous base, a binder and a filler; and a metal phosphate conversion coating formed between the back plate and the non-asbestos friction material. After a rusting test, the friction member according to this aspect of the invention has a breaking load, as measured in accordance with JIS D4422, of at least 7 MPa, and has a percent rusted surface area, defined as the ratio of the rusted surface area on the back plate to the total surface area of the plate, of at most 10%.
In a fourth aspect, the invention provides a friction member having a back plate; a non-asbestos friction material bonded to a surface of the back plate and produced by molding and curing a composition that includes a fibrous base, a binder and a filler; and a zinc phosphate conversion coating formed between the back plate and the non-asbestos friction material. In this aspect of the invention, the conversion coating is composed of crystals having a maximum length of 0.1 to 50 xcexcm and an aspect ratio of 1/1 to 1/10, has a coating weight of 0.5 to 20 g/m2, and has a phosphophyllite ratio P/(P+H), based on phosphophyllite crystals (P) and hopeite crystals (H) therein, of 0.1 to 1.
In a fifth aspect, the invention provides a friction member having a back plate; a non-asbestos friction material bonded to a surface of the back plate and produced by molding and curing a composition that includes a fibrous base, a binder and a filler; and a calcium zinc phosphate conversion coating formed between the back plate and the non-asbestos friction material. In this aspect of the invention, the conversion coating is composed of crystals having a maximum length of 0.1 to 50 xcexcm and an aspect ratio of 1/1 to 1/10, and has a coating weight of 0.5 to 20 g/m2.
In one embodiment of the friction member according to the above first, second or third aspect of the invention, the conversion coating is a zinc phosphate conversion coating which is composed of crystals having a maximum length of 0.1 to 50 xcexcm and an aspect ratio of 1/1 to 1/10, has a coating weight of 0.5 to 20 g/m2, and has a phosphophyllite ratio P/(P+H), based on phosphophyllite crystals (P) and hopeite crystals (H) therein, of 0.1 to 1.
In a second embodiment of the friction member according to the first, second or third aspect of the invention, the conversion coating is a calcium zinc phosphate conversion coating which is composed of crystals having a maximum length of 0.1 to 50 xcexcm and an aspect ratio of 1/1 to 1/10, and has a coating weight of 0.5 to 20 g/m2.
In a sixth aspect, the invention provides a method of manufacturing friction members, which method includes the steps of blasting a back plate to abrade it and remove rust therefrom; forming a metal phosphate conversion coating on a surface of the blasted back plate; and bonding to the conversion treated surface of the back plate a non-asbestos friction material produced by molding and curing a composition that includes a fibrous base, a binder and a filler. In the method of the invention, (a) the surface of the back plate is prepared with a surface preparation agent prior to the conversion coating formation step, (b) a conversion treatment solution containing a reaction modifier is used in the conversion coating formation step, or both (a) and (b) are carried out.
In the foregoing friction member manufacturing method of the invention, the surface preparation agent is preferably a titanium colloid-based formulation having an alkalinity of 0.5 to 7 points. Typically, at least one ionic species selected from the group consisting of Ni2+, Fxe2x88x92 and Mn2+ is used as the reaction modifier to form a zinc phosphate conversion coating, or Ni2+ or Mn2+ is used as the reaction modifier to form a calcium zinc phosphate conversion coating.
The blasting step in the method of the invention is preferably carried out by using high-pressure air to blow a slurry composed of an aqueous degreaser, an aqueous cleaner and particles of a greater hardness than the back plate surface against one surface of the back plate to be bonded with the friction material, or against all surfaces of the back plate, so as to abrade and remove rust from the plate.
In the practice of the invention, by forming a zinc phosphate or calcium zinc phosphate conversion coating having a small crystal size, an optimized crystal shape, and a low coating thickness, and thus a low coating weight, between a back plate and a non-asbestos friction material which is bonded to a surface of the back plate and produced by molding and curing a composition made up primarily of a fibrous base, a binder and a filler, there can be obtained high-quality friction members endowed with both excellent resistance to rusting and excellent adhesion even in a poor environment under high temperature and high humidity. Such friction members of the invention, when new, have a breaking load, as measured in accordance with JIS D4422, of at least 9 MPa and a percent bonded surface area, defined as the ratio of the surface area of friction material attached to the back plate surface after shear to the surface area of friction material attached to the plate surface before shear, of at least 90%; after a 24-hour thermal history at 300xc2x0 C., have a breaking load of at least 6 MPa and a percent bonded surface area of at least 50%; and after a rusting test, have a breaking load of at least 7 MPa and a percent rusted surface area, defined as the ratio of the rusted surface area on the back plate to the total surface area of the plate, of at most 10%.